A solid oxide fuel cell (SOFC) is a high-efficiency electrochemical power generation system which converts chemical energy of fuel gas directly into electrical energy, and uses an ion conductive solid oxide as an electrolyte. In addition, a solid oxide electrolysis cell (SOEC) based on the reverse reaction process of the above process uses the surplus power to produce chemical fuel.
Solid oxide cells such as a solid oxide fuel cell and a solid oxide electrolysis cell may share various technological bases with each other, including a device structure, operation method, or the like. In other words, most part of the materials and processing technologies of a fuel cell may be used directly for an electrolysis cell.
A single cell of a solid oxide fuel cell and electrolysis cell is composed of the electrolyte, fuel electrode and air electrode. In the case of a solid oxide fuel cell, oxygen and fuel are supplied to the air electrode and fuel electrodes, respectively, so that reduction of oxygen occurs at the air electrode to produce oxygen anions which are transferred to the fuel electrode through the electrolyte, and the fuel reacts with oxygen ions to cause oxidation into water at the fuel electrode while releasing electrons to the external circuit to produce electric power. In the case of a solid oxide electrolysis cell, when hot steam is supplied to the fuel electrode and electricity is applied, water is electrolyzed to produce hydrogen and oxygen and oxygen ions are transferred to the air electrode through the electrolyte, so that pure hydrogen is produced at the fuel electrode and oxygen is produced at the air electrode, separately.
All the cell components of solid oxide fuel cells and solid oxide electrolysis cells, including electrolyte and electrodes, possess excellent thermal properties. In addition, since such a solid oxide fuel cell and solid oxide electrolysis cell are operated at high temperature, they have higher efficiency and quality as compared to a low-temperature fuel cell and electrolysis cell.
However, the solid oxide fuel cell and solid oxide electrolysis cell should be exposed to high temperature for a long time when they are manufactured and operated, which causes various deterioration phenomena. Thus, it is difficult to commercialize such cells. More particularly, chemical reaction between an air electrode layer having a composition, such as Ba1-xSrxCo1-yFeyO3 (BSCF) or La1-xSrxCo1-yFeyO3 (LSCF), and a zirconia-based electrolyte layer causes formation of insulating reaction products at the interface, resulting in an increase in resistance and degradation of the cell quality. Moreover, degradation of thermo-mechanical stability may occur due to a difference in thermal expansion coefficients between the air electrode/electrolyte and reaction products.
Therefore, many studies have been conducted for preventing the reaction between an air electrode layer and an electrolyte layer in order to improve the quality and stability of a solid oxide cell, such as a solid oxide fuel cell or solid oxide electrolysis cell. Particularly, there is a method for forming a ceria-based diffusion barrier layer between an air electrode and electrolyte. However, a ceria-based material has poor sintering property and requires a high-temperature process at 1400° C. or higher to accomplish densification. In this case, the ceria-based material may cause chemical reaction with a zirconia-based electrolyte and thus requires sintering at a temperature of 1250° C. or lower. However, in this case, a densification degree may be degraded and a significant amount of pores is formed. Thus, it is not possible to prevent the diffusion of some of the elements from the air electrode layer to the electrolyte layer through the pores.
Under these circumstances, it is required to develop a technology which allows sintering at a temperature of 1250° C. or lower to form a dense diffusion barrier layer with high bonding strength to inhibit the diffusion of Sr from a positive electrode (air electrode), and ultimately improves the efficiency and stability of a solid oxide cell.